1. Field of the Invention
The invention relates to radio frequency identification tags or labels and in particular to a cloaking circuit used to assist in the read operations of RFID transponders.
2. Description of the Prior Art
In U.S. Pat. No. 5,963,144, incorporated herein by reference, what is described is an antenna of an RFID tag or label which is disconnected from the balance of the RFID chip by means of a series switch activated in response to a logic command, CLOAK, generated by the RFID chip. Activation of the switch disconnects the antenna of the RFID tag from the remainder of the RFID chip and effects a high impedance resistance across the antenna terminals. An RC circuit is charged by activation of the CLOAK signal and thereafter discharges during a predetermined RC time period as determined by a high impedance series antifuse leakage transistor. The antenna is thus disconnected for a time sufficient to allow the remaining RFID tags in an RF interrogation field to be identified. Meanwhile, during the disconnection of the antenna from the RFID chip and its loading causes its effective absorption and scattering aperture to be reduced to near zero so as to electromagnetically remove the RFID tag from the zone of interrogation during the predetermined time period. Hence, the interrogated tag remains disconnected and noninterferring with the RF field used to interrogate the remaining tags. It is also known to detune the tag's antenna by at least partially shorting out the antenna, as either a means of signaling or for depleting the energy stored in the antenna resonant structure.
Thus what is taught is disconnecting the front end of the RFID chip even during times when the power to the tag has been removed. This could be accomplished by open circuiting the data path and/or the power input. Unfortunately, this means that while the chip is in the Cloak state, it is impossible for the reader to communicate with the tag.
As an example, a tag with a cloak time of 20 seconds might be interrogated and then cloaked while on a conveyor belt. But it might be necessary to read the label subsequently when the tag has traveled further down the conveyor belt but within the 20 second time period.
What is needed is a circuit and method that allows a tag to be cloaked, but still to be interrogated when it is cloaked.